Oxidation and corrosion of substrates through pores in protective coatings is a serious problem in the electronics and jewelry industries. Unfortunately, there are no currently available methods for detecting substrate oxidation, and coating porosity is difficult to evaluate by known methods, such as nitric acid or electrochemical spot testing, which are generally qualitative, destructive, and hard to perform. Vacuum surface analysis techniques do not provide unambiguous results for such measurements, and they are sufficiently expensive and cumbersome to preclude their routine use for most applications. Electrochemical techniques involving the passage of current, such as sequential electrochemical reduction analysis ("SERA") for solderability testing, have limited utility in analyzing substrate oxidation. A basic difficulty with current passage techniques for analyzing protective coatings is uncertainty regarding the area of the substrate exposed to the electrolyte solution. This area uncertainty translates to uncertainty in the applied current density, which introduces errors into the analytical results. For noble metal coatings, interfering reactions such as hydrogen evolution, for example, introduce additional errors in the results.
In Morrissey, R. J., "Electrolytic Determination of Porosity in Gold Electroplates," J. Electrochem. Soc., Vol. 117, No. 6, pp. 742-47 (June 1970), it was shown that an absolute measure of porosity in terms of the area ratio of the substrate and coating exposed to a mildly corrosive electrolyte solution can be obtained if the transfer coefficients and exchange current densities are known for the reactions occurring on the two metals. These parameters can be determined from measurements of current as a function of electrode potential for the anodic substrate dissolution process and for the cathodic process (oxygen or proton reduction, for example) occurring on the coating metal. Alternatively, potential measurements can be calibrated in terms of the coating/substrate area ratio by measuring the potential difference between specimens of the coating and substrate metals having known areas. Typically, the area of the coating specimen is held constant while that of the substrate specimen is varied. For the particular electrolyte (0.1M NH.sub.4 Cl) and stagnant solution used by Morrissey, long times of 10 to 100 minutes were required to reach a steady-state potential. Experiments recently performed by Applicants suggest that potential measurements made using stagnant electrolytes do not provide an accurate measure of coating porosity in most cases.
Because of the limitations of the prior art, there is a need for a rapid, practical, and quantitative method for determining porosity of protective coatings and oxidation of the underlying substrate. Such a method would lead to improvements in coating processes and quality control. It would be particularly beneficial for evaluating protective coatings, such as gold, palladium, or organic coatings on nickel or copper, that are used in the electronics and jewelry industries.